This invention relates to pipe laying apparatus, and, more particularly, to a pipe laying apparatus for laying pipes of a relatively small diameter underground.
Recently, for laying pipes of a small diameter of, for example, less than 800 mm, underground, a propulsion process has become more popular than an open-cut or open trench process. In the propulsion process a pipe to be laid underground is propelled by pushing trailing end thereof by propulsion means such as a hydraulic cylinder located in a starting pit, and moving the pipe forwardly while forcing the earth ahead of the pipe to be compacted to lay the pipe underground. This type of propulsion process is referred to as a compaction system; however, a disadvantage of the compaction system resides in the fact that the earth offers great resistance to the pipes to be laid that are moved forwardly because they are merely pushed at their trailing ends by the hydraulic cylinder, so that a propelling force of high magnitude is required to move the pipes forwardly. Moreover, since a large force is exerted on the pipes, the pipes are subject to being damaged. Also, the system offers the disavantage that it is low in directional precision because the pipes laid by this system might be displaced from the direction in which they are intended to move.
To avoid the aforesaid disadvantages, proposals have been made to use a propulsion process of a rotary excavation system in which pipes are propelled by means of a hydraulic cylinder while a hole is being excavated by a rotary excavator to lay the pipes underground. Japanese Patent Laid-Open No. 29797/82 describes a pipe laying apparatus having particular utility in carrying out the propulsion process of the rotary excavation system for laying pipes underground.
The apparatus described in the above-noted Japanese Publication comprises a hydraulic cylinder, serving as propulsion means, mounted in a starting pit, drive means for driving rotary excavating tools for rotation, and viscosity imparting means. An excavator is provided which includes an excavator main body rotatably supporting at its leading end the rotary excavating tools which have a greater outer diameter than pipes to be laid and are formed with a port for injecting a viscosity imparting liquid into the earth. The rotary excavating tools comprise excavating cutters and agitating blades. The pipes to be laid are connected at their leading end to the trailing end of the excavator main body and the trailing end of the pipes is positioned against the hydraulic cylinder. Extending through the interior of the pipes is a hollow rotary shaft for the rotary excavating tools formed in the interior with a passageway for the viscosity imparting liquid to flow therethrough. The rotary shaft is connected at one end thereof to the rotary excavating tools and at other ends thereof to the drive means for driving the rotary excavating tools and the means for supplying the viscosity imparting liquid, respectively. A pressure bearing frame for holding the pressure of the soil particles is mounted in an annular gap defined between a horizontally extending hole formed by excavation and the pipes to be laid at an end thereof which opens in the starting pit. The pressure bearing frame is formed with a discharge opening.
The drive means for the rotary excavating means and the means for supplying the viscosity imparting liquid are actuated resulting in a rotating of the rotary excavating tools to dig a hole by the excavating cutters while the viscosity imparting liquid is injected through the injecting port into the earth dug and broken into soil particles, so that the soil particles and the viscosity imparting liquid are mixed and agitated by the agitating blades to produce viscosity imparting liquid containing soil particles. Since the rotary excavating tools have a greater outer diameter than the pipes to be laid, and an annular gap is defined between a substantially horizontally extending hole formed by excavation and the pipes laid underground. The viscosity imparting liquid containing soil particles, produced in the vicinity of the rotary excavating tools, are conveyed rearwardly of the excavator by the pressure under which the viscosity imparting liquid is injected into the earth and the propelling force of the hydraulic cylinder exerted on the pipes. Thus, the viscosity imparting liquid is moved past an outer periphery of the excavator main body and through the annular gap and the discharge port, to be ejected into the starting pit. Meanwhile, the hydraulic cylinder has its piston rod extended to push the pipes forwardly in the earth as excavation is performed by the excavating tools. When the piston rod of the hydraulic cylinder reaches the end of its stroke, the piston rod is returned to a contracted position and a new pipe is connected to the trailing end of the pipes laid in the starting pit. The aforesaid operation is repeated to successively lay one pipe after another underground.
In the pipe laying apparatus of the aforesaid construction and operation, the earth is excavated to produce the viscosity imparting liquid in the forward end portion of the excavator main body. This offers the advantage that the resistance offered to the forward movement of the pipes by the earth is greatly reduced. Moreover, since the annular gap between the horizontally extending hole formed by excavation and the pipes is filled with the viscosity imparting liquid, friction between the pipes and the earth is greatly reduced. Thus, the pipe laying apparatus offers the advantages that the propelling force exerted in the pipes by the propulsion means can be reduced, damage to the pipes can be minimized because the force exerted is reduced, and directional precision can be improved, as compared with pipe laying apparatus of the compaction system.
One disadvantage of the last described pipe laying apparatus resides in that fact that, since the viscosity imparting liquid is conveyed through the annular gap between the horizontally extending hole formed by excavation and the pipes laid toward the starting pit, the annular gap increases in length when the number of pipes laid increases and the distance to be covered by the forward movement of the pipes becomes greater, so that the resistance offered to the viscosity imparting liquid moved rearwardly through the annular gap increases. Thus, it is necessary to increase the propelling force exerted by the propulsion means on the pipes to a level high enough to enable the viscosity imparting liquid to be conveyed toward the starting pit by overcoming the resistance offered to their movement through the annular gap, although it would not be necessary to increase the propelling force to the same level as that exerted on pipes in apparatus of the compaction system.